You load up the parameters into an OleDbCommand instance, and then throw it all away to create a new, empty instance and store it in the same variable.

To be honest, if you are "just a beginner who learns through some tutorials" then you are doing it all wrong, particularly if these are YouTube tutorials - I've yet to see one that is of any real use whatsoever. Instead, look for a good book on the subject - Apress, Wrox, Addison Wesley, Microsoft Press - they all do excellent beginner volumes though I don't know if any of them are available in French. If you can, look for a copy of "Pro C# 8.0" (APress, I believe), or "C# in a nutshell" (O'Reilly?) - I learnt from those one many, many years ago when .NET was at V2!
Books introduce the material in a structured way, building on what has been taught before - and aren't written just to get views and subscribers...

"I have no idea what I did, but I'm taking full credit for it." - ThisOldTonyAntiTwitter: @DalekDave is now a follower!

really thank you for your advice i will inquire to see if the book is available in french. I did a bit of programming at school but not the csharp, and since it is a langauge that I am passionate about, I decided to take it easy but I really want to know the role of each method used. And for its I thank you again because I followed your advice. But with your permission I would like to send you some project that I have realized thanks to courses on Csharp to just give me your opinion on the presentation of my codes and if there is an improvement to be made.

Don't send it, I won't look at it.
I don't have time to be a mentor to anyone - and I get a couple of request for it a month, and certainly couldn't do it for everyone who asks: so I don't do it at all to be as fair as I can to everybody.

"I have no idea what I did, but I'm taking full credit for it." - ThisOldTonyAntiTwitter: @DalekDave is now a follower!

It will (1) be able to catch more problems, and (2) will provide much more information when something goes wrong (including the exact line number when running in Visual Studio); you may not understand all of its output right away, but that typically is the info one needs to easily pinpoint what went wrong.

Hello sir and thank you for the info, I think I saw it when I was doing my research. And it was written only to see the problems with connection to the database. So I would like to know if we could use it only at the connection of the database or not?

Throws the sql_cmd variable away and sets it to a new OleDbCommand instance;

Attempts to execute the sql_cmdwithout adding any parameters to it;

This is yet another reason not to store the OleDbConnection and OleDbCommand objects in class-level fields. The first bit of your code is manipulating a command from a previous method. If the sql_cmd field hasn't been initialized, you may even get a NullReferenceException.

Change your code to create and use a new OleDbCommand instance, wrapped in a using block: